1. Field of the Invention
This invention relates to a method of forming a spark plug cap with a capacitor for detecting ignition voltage, more particularly to a method for forming such a spark plug cap with a capacitor for use in detecting misfire occurring during operation of a gasoline engine or other spark-ignition type internal combustion engine.
2. Description of the Prior Art
As is well known, in gasoline and other spark plug ignition combustion engines a high voltage produced by an ignition coil is supplied to the spark plugs at the engine cylinders. At each cylinder, the resulting spark discharge between the spark plug electrodes produces a spark which ignites an air-fuel mixture that has been drawn into the cylinder, causing the mixture to burn explosively. In the course of this ignition-combustion process in the internal combustion engine, the mixture may for some reason occasionally fail to burn properly. This is referred to as a misfire. Misfires can result from causes in either the fuel supply system or the ignition system. Misfires caused by problems in the fuel supply system are the result of an overly lean or overly rich air-fuel mixture. In that case, a spark is produced between the spark plug electrodes but the air-fuel mixture does not ignite. Misfires caused by problems in the ignition system are the result of spark plug electrode fouling or ignition circuit malfunctions which prevent normal spark discharge.
The occurrence of misfire in the course of engine operation not only degrades driving performance but also lowers fuel efficiency and may cause after-firing of unburned gases in the exhaust system, which can damage the exhaust gas after-treatment system and have other adverse effects. Moreover, since the occurrence of even a single misfire indicates a misadjustment or malfunction in the fuel supply or ignition system, prompt elimination of the problem is essential. Because of this, there is a strong need for development of a system for detecting misfires as soon as they occur.
One type of misfire detector that has been proposed is the mis-spark detector described in Japanese Laid-Open Patent Publication No. 52(1977)-118135. As shown in FIG. 21, the detector includes a conductor 51 wrapped around a portion of a high-voltage cable 50 of the engine ignition system so as to constitute a detective capacitor 52 (a type of capacitance probe) in which the insulation cladding 50A of the high-voltage cable 50 serves as the dielectric. A voltage divider capacitor 53 is connected between the capacitor 52 and the ground so that the ignition voltage (secondary voltage of the ignition coil) applied to the conductive core 50B of the high-voltage cable 50 induces a voltage across the terminals of the capacitor 52 owing to its static capacitance. The induced voltage is statically divided by the detection capacitor 52 and the divider capacitor 53, and the voltage across the terminals of the divider capacitor 53 (the divided voltage) is forwarded as a detection voltage to an electronic circuit 54 for processing and discrimination. The electronic circuit 54 detects the occurrence of misfires from the difference between the wave form of the ignition voltage at the time of normal spark discharge and that at the time of no spark discharge (missparking). Among the different types of misfires, the detector thus detects misfires that occur when no spark discharge is produced owing to a problem in the ignition system.
Another detector for detecting misfire in internal combustion engines is disclosed in the assignee's Japanese Laid-Open Patent Publication No. 5(1993)-65868. In this detector, the ignition voltage is similarly detected from a high-voltage cable or the like of the ignition system using static voltage division, and misfire owing to causes in the fuel supply system is detected based on the fact that, even when spark discharge occurs, the wave form of the ignition voltage differs between the case where normal combustion occurs and the case where it does not.
Thus in conventional misfire detectors, such a capacitance probe is used as a means for detecting ignition voltage. The probe is constituted by wrapping a sheet or ribbon of conductor around the high-voltage cable of the ignition system so as to form a detective capacitor between the conductor and the core of the high-voltage cable using the insulation cladding of the high-voltage cable as the dielectric. However, the capacitance probe constituted in this manner has a major drawback that derives from the nature of the high-voltage cable of the ignition system. Because of its flexibility and elasticity, the high-voltage cable is highly susceptible to vibration. It is also easily affected by changing ambient humidity, wetting by leaking water, and fouling with oil, grime and the like. When a capacitor for use in detection is formed by wrapping a conductor ribbon around the cable, the static capacitance of the capacitor is apt to be changed from its proper value by a shifting of the conductor caused by vibration as well as by changing humidity, wetting with water and fouling with oil, grime and the like. Although some change in static capacitance can be tolerated if the capacitor is to be used only for checking the ignition voltage, even slight changes have to be avoided when it is used for misfire detection because such detection generally requires accurate detection not only of the ignition voltage but also of the ignition voltage wave form. The capacitance changes to which the prior art capacitance probe is susceptible may alter the detected voltage wave form and thereby make it impossible to detect misfire with high reliability.
In actual practice, moreover, the flexibility and elasticity of the high-voltage cable make the work of attaching the conductor for forming the capacitor on the insulation cladding of the high-voltage cable and securing it thereon difficult and troublesome. Maintenance of the so-formed capacitor is also troublesome.
In addition, the insulation cladding of the high-voltage cable is generally formed of synthetic rubber, a material that is readily degraded when exposed to heat and/or fouled with oil and grime. This degradation of the insulation cladding after the detective capacitor has been formed by winding the conductor around the high-voltage cable not only produces a progressive change in the static capacitance of the capacitor over time but may also reduce the electric insulation strength of the cladding to the point that the high ignition voltage can leak to the conductor wrapped around it. When this happens, the high leak voltage is apt to be conducted to the electronic circuitry of the misfire detector, which it can damage or cause to malfunction. Ideally, moreover, for enabling accurate detection of the ignition voltage wave form, the distance between the conductor and the conductive core constituting the opposite sides of the detection capacitor should be made as small as possible so as to increase its static capacitance. In actuality, however, the conductor and the cable conductive core have to be spaced at a greater distance than desirable as a precaution against loss of the insulation property in the cable cladding owing to the aforesaid causes as well as to corona discharge. As a result, it has been impossible to avoid low accuracy in detecting the ignition voltage wave form.
For overcoming these problems, in Japanese Patent Application 4(1992)-56395; also filed in the United States on Feb. 2, 1993 under the number of 08/012524, the assignee proposed a capacitor for ignition voltage detection in which the conductor is provided in a spark plug cap inserted into the secondary circuit of the ignition system. The ignition plug cap is fixed to cover the terminal portion of an internal combustion engine spark plug. It is equipped with a high-voltage conductor for conducting high voltage for spark discharge to the terminal portion and the high-voltage conductor is enclosed with an insulator. A conductor for detecting ignition voltage is provided integrally with the insulator at a prescribed distance outward from the surface of the high-voltage conductor.
With this configuration, since the detective conductor constituting a part of the detective capacitor for detecting the ignition voltage is provided integrally with the structurally sturdy insulator of the spark plug cap, it has excellent structural durability. Moreover, differently from the case where the detective conductor is provided on a high-voltage cable of the ignition system as in the conventional arrangement, there is no danger of the position of the detection conductor shifting because of mechanical vibration or of it being affected by changes in the ambient humidity, wetting by leaking water or fouling with oil or grime. Since it is therefore possible to prevent such causes from producing a change in the static capacity of the detective capacitor, the wave form of the ignition voltage can be accurately detected at all times. As a result, the presence/absence of misfire can also be detected with high accuracy. Moreover, the detective conductor requires no maintenance once it has been installed.
The aforesaid configuration in which an ignition voltage detection capacitor is integrally formed in the interior of a spark plug cap has numerous advantages over the conventional configuration in which the detective capacitor is formed in the high-voltage cable. However, it has a drawback when actually applied to a spark plug cap intended for volume production.
The simplest method of fabricating a spark plug cap to have the structure described above is to mold the insulator (body) enclosing the high-voltage conductor of insulating resin and at the time of molding the insulator simultaneously embedding the detective conductor in the resin. When this method is used, however, the molding has to be conducted with the detective conductor suspended in the space within the mold. This type of molding is extremely difficult to conduct in actual volume production.